Abstract: A method of preparing a product, preferably in the form of a functionalized high molecular weight component such as, for example, a functionalized protein includes passing a first liquid comprising a component A along one side of a semi-permeable membrane and passing a second liquid comprising a component B along another, preferably opposing, side of the membrane, wherein the membrane excludes passage of component A but allows passage of component B, and after passage of component B through the membrane, a chemical reaction takes place between component A and component B, preferably with the formation of covalent bonds.